Plant Physiology/Ecology

Given that rainfall is the key resource driving net primary production, it is vital to develop a clear understanding of how many common plant species respond to changing patterns of summer rainfall. Our rainfall manipulation experiments provide an ideal setting to study the ecophysiological responses of dominant and subordinate grasses to altered patterns of monsoon rainfall under relatively controlled experimental conditions. (mentor: Scott Collins)

Pulsed precipitation is a defining feature of semiarid ecosystems, these wetting and drying cycles promote different plant physiological responses to precipitation. When it rains, plants decide if its worth investing energy to take advantage of a particular precipitation pulse (e.g. by producing new roots or synthesizing new enzymes for photosynthesis) with the extent of response being highly dependent on the level of stress attained during dry inter-pulse periods and the plant functional type composition. Investigating the relative importance of these adaptations is of primary interest to understand the functioning of Piñon and Juniper woodlands, an extensive ecosystem type in the semiarid North American Southwest. Physiological responses to precipitation pulses can be gauged with measurements of photosynthesis and transpiration along with simple models of plant function. Within the context of a large-scale rainfall manipulative experiment in a PJ woodland at the Sevilleta LTER, we provide the opportunity to develop experiments based on these gas exchange and modeling techniques to asses the physiological response of trees to wetting and drying cycles. (mentor: Will Pockman)

Understanding water dynamics in the rhizosphere is key to developing ecological theory on how plants adapt and survive harsh conditions. Due to the scarcity of water in semiarid ecosystems, gradients of soil water availability develop in space and time. Investigating the strategies that plants develop to optimize the use of available water in the soil is a very active topic of research in ecology. We invite students to develop experiments to characterize some of the belowground processes that Piñon and Juniper trees display to cope with water scarcity and how such strategies affect water dynamics in the rhizosphere. Measurements of root hydraulic conductivity, root sap flow, and plant water potential among other techniques will be available to formulate such ecohydrological experiments within the framework of a large-scale experimental setting at the Sevilleta LTER.  (mentor: Will Pockman)

Determination of Pinon-Juniper Foliage Biomass on the Sevilleta. Quantification of annual aboveground net primary production among trees in Pinon-Juniper (PJ) habitats is challenging Our past methods for annual foliage production as well as wood increment addition on both Pinon (Pinus edulis) and Juniper (Juniperus monosperma) lack the precision necessary for detecting inter-annual variation. . Most recently, dendrometers (bole growth measuring devices) have been installed on a number of individual trees at a Sevilleta PJ site that will allow a better measurement of annual gross growth. However, we now need a method of relating such bole growth to annual foliage production for both species. This proposed project would involve developing methods for this and testing their efficiency under field conditions. (Mentor: Doug Moore/E. Muldavin)